Over the 30 years' duration of this grant, this project has established a broad foundation for understanding the role of carbohydrate structures on glycoproteins. In this time our studies have defined 1) unique oligosaccharide structures as signatures found on specific plasma glycoproteins, 2) the mechanisms of recognition of these structures by carbohydrate-specific endocytic receptors, and 3) the functionally important in vivo biological consequences of this recognition. We have proceeded from determining at the molecular and cellular level the biologic basis of oligosaccharide recognition by these receptors, to delineating at the whole organism level the functional implications of such recognition. Our progress on how carbohydrate-specific endocytic receptor recognition systems clear particular glycoproteins from the circulation now culminates in a unique opportunity to take these studies to a new level of functional perspective. We have demonstrated by biochemical and genetic means that the Mannose/GalNAc-4-SO4-receptor (M/G4S-R) and the asialoglycoprotein-receptor (ASGP-R) mediate the respective clearance of glycoproteins that bear the N-linked oligosaccharide signatures of terminal GalNAc 4 SO4 and of terminal Sia12,6GalNAc. We have shown that the ASGP-R can bind glycoproteins bearing Sia12,6Gal. We hypothesize that glycoproteins bearing terminal Sia12,6Gal are in fact the major endogenous ligands for the ASGP-R and that the ASGP-R is a key regulator of the concentration of numerous plasma glycoproteins that have Sia12,6Gal termini. Preliminary studies support this proposed function. Our findings give a significant new functional identity to this receptor. We will now use genetic models and recently established proteomic methods to define the relative roles of the ASGP-R and M/G4S-R. We will determine how these receptors act, individually and in combination, to regulate levels of plasma glycoproteins based on their carbohydrate signatures, under physiologic as well as pathologic conditions ranging from regulation of estrogen production by pituitary glycoprotein hormones to the acute phase response to sepsis. These proposed studies utilize single and double receptor knockout mice 1) to address how production is balanced with clearance to regulate plasma levels of glycoproteins through recognition of their carbohydrate signatures, 2) to determine if perturbing this balance has an impact on biologic processes, and 3) to establish the molecular basis for generating isoforms of the M/G4S-R with differing carbohydrate signature specificities and determine the impact of such receptor isoforms on reproductive biology. The fundamental mechanistic information that we uncover will ultimately yield insights that can aid in evaluating changes in plasma proteins for both diagnostic and therapeutic purposes. Public Health Relevance: The levels of glycoproteins in the blood reflect a balance between their synthesis and removal. We have identified two major carbohydrate specific receptor systems in liver that help control the concentrations of many glycoproteins in the blood by determining their rates of removal. We will define how these receptors control the concentrations of hormones and inflammatory proteins during pregnancy and the response to injury.